1. Field of the Invention
The present invention relates to an optical disk playback apparatus and an optical disk recording/playback apparatus whereby a unipolar RFDC signal is balanced-modulated and fed to a data-reproducing RFAC section for demodulation in order to acquire an off-track signal or a modulation factor.
More particularly, the invention relates to an optical disk playback apparatus and an optical disk recording/playback apparatus for causing an RFAC-based AGC (Automatic Gain Control) to demodulate the balanced-modulated RFDC signal into a demodulated RFDC signal before normalizing the demodulated signal by reflectance in order to suppress those variations in modulation factor measurements which are caused by the variations in reflectance.
2. Description of the Related Art
In general, the optical disk playback apparatus and optical disk recording/playback apparatus use an OFTRK (off-track) signal in order to apply pulling brakes at seek time. The OFTRK signal is a signal that binarizes the bottom level, which shows an opposite change to an absolute value of the off-track amount, in reflectance of the mark strings recorded on the optical disk. As such, the OFTRK signal has a phase difference of 90 degrees relative to tracking errors. The OFTRK signal and a TEZC signal (tracking error binarizing signal) are AND'ed to generate a brake signal. A servo controller uses this brake signal as a gate signal by which to let a brake current flow through the tracking coil.
Generally, the optical disk recording/playback apparatus uses the modulation factor of the mark strings recorded on the optical disk as a control index for OPC (optimum power control). Parenthetically, there are two types of optical disks: those on which the reflectance of recorded marks drops, and those on which the reflectance rises. The present invention applies to both types of disks. Thus it is assumed for the ensuing description that the reflectance of the marks on the disk drops.
The modulation factor of the mark strings is calculated using the following formula: [(space reflectance−mark reflectance)/space reflectance]. Space reflectance is equal to the reflectance of unrecorded disks.
The OFTRK (off-track) signal and modulation factor need to be acquired from the RFDC signal in which space reflectance and mark reflectance are preserved. Usually, one of the following two methods has been adopted to obtain the OFTRK signal and modulation factor from the RFDC signal:
(1) As shown in FIG. 6, an ordinary optical disk recording/playback apparatus 300 inputs the RFDC signal to high-speed wideband HP (peak hold)/BH (bottom hold) circuits whose outputs are subjected to calculations for obtaining the OFTRK signal and the modulation factor. The PH value stands for space reflectance and the BH value for mark reflectance. More specifically, the OFTRK signal is acquired by binary slicing the output signal of the BH circuit, and the modulation factor is obtained by performing the calculations of [(PH output signal−BH output signal)/PH output signal].
(2) As shown in FIG. 7, an ordinary optical disk recording/playback apparatus 400 digitizes the RFDC signal through an RFAC-based analog/digital converter (ADC) before inputting the resulting digital signal to PH and BH circuits. The process outlined in the preceding paragraph (1) is then digitally carried out to acquire the OFTRK signal and modulation factor.
In connection with the related art, the reader is requested to refer to Japanese Patent Laid-open Nos. Hei 8-111030, 2001-351249, and 2004-253016.